Well, I think my English was not so good, because you don't answer to my problem.

I have a robots.txt at root, the default robots.txt we get when installing Drupal.
Just 1 robots.txt at root.

Question is why robots are sometimes looking to ....de/robots.txt?

I guess it's because of following fact :
I have a second domain name deutschname.com which is redirected to mysite.com/de/
So I guess when robots are looking for deutschename.com/robots.txt, they are redirected to mysite.com/de/robots.txt.
Can you confirm it's surely the reason why scrawler are looking for mysite.com/de/robots.txt ?

Perhaps, the fact that scrawlers do not find mysite.com/de/robots.txt is not a problem and I must let it down.
Can you confirm?

How can I know for sure that robots are looking for deutschename.com ?
Is not it best that robots do not scrawl at all for deutschename.com?
How can I stop it? Will it penalize the SEO for German language?

It's a lot of cascading questions, but at least for me, it makes sense to ask all of them.